The present invention relates to a speed control device for a DC motor which utilizes a DC motor speed controlling circuit such as an IC circuit to control the speed of a DC motor.
In general, the speed of a DC motor may be controlled by comparing a voltage which is proportional to the rate of rotation or speed of the motor with a reference voltage. A DC motor utilizing a constant strength field can be equivalently represented by a series combination of its internal resistance R.sub.o and a counter electromotive force E.sub.o induced by rotation of the DC motor. When a current I.sub.a flows in the DC motor, the relation of the rate of rotation to the counter electromotive force E.sub.o is: EQU N=E.sub.o /K.sub.2 Z.PHI. (1)
where K.sub.2 Z.PHI. is the counter electromotive constant. On the other hand, the relation of the load torque T.sub.d to the current I.sub.a of the DC motor is: EQU T.sub.d =K.sub.1 Z.PHI..multidot.I.sub.a ( 2)
where K.sub.1 Z.PHI. is the torque constant. Furthermore, the terminal voltage V.sub.m of the DC motor can be represented by: EQU V.sub.m =E.sub.o +R.sub.o .multidot.I.sub.a. (3)
Equation (3) can be rewritten as E.sub.o =V.sub.m -R.sub.o .multidot.I.sub.a. As can be seen from the equation, a voltage corresponding to the speed of rotation of the motor can be detected. The detected voltage may be coupled back to a control system which in response thereto causes the DC motor to run at a constant speed irrespective of variations in the load applied to the motor.
This principle of operation is well known in the art. In order to implement this principle, it has been the practice to use an integrated circuit for the control system. As disclosed in Japanese Published Patent Application No. 58289/1977, for instance, an IC for controlling the DC motor speed is typically constructed as a three-terminal device and is connected to a small number of external passive circuit elements. An example of such a DC motor speed control device is shown in FIG. 1.
In FIG. 1, reference character M designates a DC motor; 1 a speed controlling IC; R.sub.t and R.sub.s resistors; Q.sub.1 and Q.sub.2 transistors; (1), (2) and (3) terminal pins, namely, an input terminal, an output terminal and a reference terminal, respectively; I.sub.a and I.sub.t currents flowing as indicated by the arrows; R.sub.o the equivalent internal resistance of the DC motor; E.sub.o the counter electromotive force of the DC motor; E.sub.ref a reference voltage; Vcc a supply voltage; and 1.sub.a an error detecting circuit.
With the three-terminal IC as described above, a reference voltage E.sub.s which is used to set a desired rotational speed for the motor may be represented by: EQU E.sub.s =E.sub.ref +(E.sub.ref /R.sub.s)R.sub.t. (4)
The rotational speed of the motor is: ##EQU1## For a three-terminal IC of this type, the reference voltage E.sub.s and the rotational speed are varied by varying the values of the external passive circuit elements, namely, R.sub.t and R.sub.s.
In general, ferrite magnets have been employed as the field system of a DC motor. The counter electromotive force E.sub.o of the DC motor, being affected by the flux of the field system, has a negative temperature coefficient while the equivalent internal resistance R.sub.o of the DC motor has a positive temperature coefficient because of the armature winding. Accordingly, a resistor R.sub.t having a positive temperature coefficient is provided to compensate for the temperature dependency of the equivalent internal resistance R.sub.o. However, if temperature compensation is carried out only with the resistor R.sub.t, as is apparent from equation (4), the reference voltage E.sub.s must have a positive temperature coefficient making it impossible to compensate for the counter electromotive force E.sub.o having a negative temperature coefficient. Hence, the problem that the speed of rotation of the motor varies with temperature cannot be solved in this manner. If the value of the resistor R.sub.s is changed to control the speed of rotation, then the ratio of the resistance R.sub.t to the resistance R.sub.s will change as is clear from the equation (5) as a result of which the temperature coefficients will fluctuate.